In an effort to remove the black streaks on my fiberglas coach walls I have tried several different methods, none seem to work very well. Other than a full blown compound buffing and wax, here is what I just discovered.
After scrubbing with a brush using soap and water, and drying, I took a rag dampened lightly with mineral spirits and rubbed gently over the area. The streak vanished. I wiped dry and followed with a cleaner wax and the wall in that spot looks new. Hopefully the picture is clear enough to see the clean spot to the left of the area with black streaks. I did go over the spot you can see with the black streaks with cleaner wax only.